The present invention relates generally to open loop control systems for motors, particularly stepper motors, and more particularly to open loop control systems for stepper motors used in disk files and particularly where the stepper motor is utilized to position a transducer with respect to a record media.
Disk files are in wide use for the storage and reproduction of signals on magnetic disk media. Such disk media have multiple parallel circular tracks. A moveable transducer is mounted on a control arm and is capable of servicing a plurality of record tracks. A particular disk file may have a plurality of disk platters with each disk platter having two surfaces, both of which may be utilized. A particular disk file may even have multiple transducers per disk platter surface. The transducer movement with respect to the parallel circular tracks on a particular disk platter surface may be controlled by the use of a stepper motor. Upon command from a control unit, the stepper motor will move the transducer from one selected record track to another selected record track. This movement, consisting of acceleration and deceleration, is controlled by a system controller which knows which record track it is servicing and to which selected record track it is to be moved.
How fast a stepper motor, and hence a transducer, reacts to a request to change record tracks, "seek" mode, and how accurate a stepper motor holds a particular record track, "detent" mode, is extremely important in disk file applications. Accuracy in positioning is directly related to the track density which is achievable.
Great measures are taken to increase track density. For example, compensating temperature coefficients of expansion are built into transducer positioning mechanisms. Temperatures are maintained as close to constant as possible by the addition of substantial cooling mechanisms, and by the achievement of constant power dissipation even in different modes of operation. Such a constant power dissipation will produce a constant amount of heat generated from that power dissipation and will result in a more nearly stable temperature given a stable environment.
In control systems for a stepper motor, power to the stepper motor may come from a programmed current high impedance source (sometimes called a "constant current" source), from a programmed voltage low impedance source (sometimes called a "constant voltage" source), or from a source with intermediate impedance characteristics. The programmed current source and the intermediate impedance source may be achieved in a variety of ways and are well known in the art. The programmed voltage source is not commonly used. One text which is especially helpful in explaining such drive systems is entitled Incremental Motion Control--Step Motors and Control Systems, edited by Benjamin C. Kuo, copyright 1979, published by S.R.L. Publishing Company, P. O. Box 2277, Station A, Champaigne, Illinois 61820, which is hereby incorporated by reference. Of particular interest in this text is Chapter 4 relating to drive circuitry for stepper motors.
Stepper motor control systems utilizing a programmed current source are advantageous because the rate at which the stepper motor current can be changed is very fast. This means that the rate at which the sequence of current values through which a stepper motor must be sequenced, during seek mode, can be made quite rapid. The rate is limited mainly by the voltage at which the programmed current source saturates since it is this voltage which sets the rate of charge and discharge of the stepper motor winding inductance.
However, during seek mode, while a transducer is moving to a new target record track, and as that transducer is reaching the target record track, a stepper motor positioning the transducer will tend to oscillate. This oscillation manifests itself in an oscillation of the motionally induced (back) emf of the motor phase windings. This oscillation and the need to damp these oscillations is recognized in the Kuo text, especially in Chapter 8 entitled "Damping of Step Motors."
If at its final position the stepper motor is controlled by a programmed current control source, a substantial time is required to damp these oscillations because no electronic damping is available. In one exemplary system, the time to damp this oscillation has been shown to be approximately 30 cycles of the basic motor/load resonant frequency.
Some open loop control systems utilized for stepper motors used for positioning transducers use techniques to damp these inherent oscillations of the stepper motor. Techniques commonly used to damp stepper motors which are well known in the art are enumerated in Chapter 8 of Kuo's book. The mechanical dampers have the advantage of being insensitive to the phase of the oscillations occurring as the stepper motor reaches its last step, target record track, but suffer the disadvantage of high inertia, high cost, large size and poor reliability. The open loop electronic dampers suffer from the disadvantage of requiring timing which must be related to the phase of the oscillations occurring as the stepper motor reaches its last step. In fact, in random access positioning systems, considerable oscillations are present as the stepper reaches its last step. Furthermore, the phase of these oscillations depend on the number of steps, the prior speed profile, humidity and other factors making successful timing of the electronic dampers very difficult to achieve.